System and method for increasing III-nitride semiconductor growth rate and reducing damaging ion flux

ABSTRACT

Systems and methods are disclosed for rapid growth of Group III metal nitrides using plasma assisted molecular beam epitaxy. The disclosure includes higher pressure and flow rates of nitrogen in the plasma, and the application of mixtures of nitrogen and an inert gas. Growth rates exceeding 8 μm/hour can be achieved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage of International PatentApplication No. PCT/US2016/037930, filed on Jun. 16, 2016, claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/180,167,filed Jun. 16, 2015, entitled “Method of Increasing III-NitrideSemiconductor Growth Rate and reduced Damaging Ion Flux content,” theentire contents and substance of which are hereby incorporated byreference as if fully set forth below.

STATEMENT OF FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No.EEC1041895 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

TECHNICAL FIELD

The various embodiments of the disclosure relate generally to processes,methods, and systems for plasma-assisted molecular beam epitaxy (MBE).It is particularly useful for the rapid production of group III-nitridesemiconductors.

BACKGROUND

Group III-nitride semiconductors have seen enormous commercial growth inrecent years for solid state lighting and power electronics. While themajority of light emitting diodes (LEDs) are grown by metalorganicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) has seensuccess in the power electronics market for the growth of high electronmobility transistors (HEMTs)) and promises expansion of light emittersto wavelengths currently challenging for MOCVD. Plasma-assisted MBE(PAMBE) typically uses radio frequency (RF) plasma to generate reactivenitrogen species from inert nitrogen gas. PAMBE using RF plasma has beenshown to result in higher growth rates and improved surface morphologycompared to other plasma techniques owing to lower ion content andreduced surface damage during growth.

Nonetheless, PAMBE is typically performed at growth rates on the orderof 0.1 to 1 μm/hour, substantially lower than the growth rates typicallyused in MOCVD growth of GaN which commonly exceed 1-3 μm/hour. Theserelatively low growth rates limit the applicability of PAMBE for manydevice structures which require thick buffer layers to reduce defectdensities resulting from heteroepitaxy. Although the ultra cleanenvironment of MBE can be beneficial for the thick, undoped driftregions of p-i-n rectifiers or LEDs, the slow growth rates stillpreclude the use of MBE for such devices. Finally, if III-Nitrides, withtheir tunable band gap and strong light absorption, are ever to becomeviable for solar applications, thick indium-bearing layers must bedemonstrated in order to reduce defect densities. Such layers with highindium mole fractions are impractical in MOCVD due to the lowtemperature requirements for growing indium bearing alloys, while hightemperatures are required for cracking ammonia. The result for mostMOCVD growth of InGaN is very low growth rates and highly inefficientprecursor usage. Contrarily, these high indium composition alloys aredramatically more suitable for PAMBE where substrate temperatures can bereduced in order to facilitate proper indium incorporation and noammonia cracking is required. If rapid growth of indium-bearing layerscan be achieved, then the potential of III-nitrides for photovoltaicsimproves dramatically.

BRIEF SUMMARY

The various embodiments of the disclosure relate generally to processes,methods, and systems for rapidly growing Group III metal nitrides in anPAMBE process.

An embodiment of the disclosure can be a PAMBE system having a growthchamber with a substrate therein, a remote plasma chamber, and agas-conductance barrier separating the plasma chamber from the growthchamber. The plasma can contain nitrogen and a inert gas mixture. Thegrowth chamber can have a pressure P_(g), and the plasma chamber has apressure P_(p), and the gas conductance barrier allows the pressureP_(g) to be lower that P_(P).

In some embodiments, P_(P) can be at least 0.1 mTorr, at least 1 mTorr,at least 10 mTorr, or at least 100 mTorr. In some embodiments, the P_(g)can be less than about 0.1 mTorr, or less than about 0.05 mTorr. In oneexample, P_(P) can be at least 5 mTorr, and P_(g) can be less than about0.1 mTorr.

In some embodiment, the nitrogen gas flow of the plasma can be at least3 sccm based on a 2 inch diameter substrate. The nitrogen gas flow canbe scaled to larger systems based on the equationSCCM=(GR)*(AREA)/SCALE, where GR is the growth rate in μm/hour, AREA isthe size of the target in square inches, and SCALE is a factor withunits (μm inches{circumflex over ( )}2)/(SCCM-hour), wherein SCALE isbetween 0.1 and 50, between 1 and 50, or between 1 and 20.

In some embodiments, the plasma can contain nitrogen gas and an inertgas. The inert gas can be selected from helium, neon, argon, xenon, andkrypton, preferably from helium neon, argon and xenon. The inert gas canbe selected from the group consisting of argon and xenon. The plasma caninclude nitrogen gas and argon. In some embodiment, the nitrogen toinert gas ratio can be 1:20 to 20:1, or 1:1 to 10:1.

In some embodiments, the gas-conductance barrier has a conductance valueof at least about 7 L/sec, about 10 L/sec, or about 15 L/sec. In someembodiments, the gas-conductance barrier can have a conductance value ofat least about 5 L/sec or 10 L/sec. The conductance value can be scaledby r², where r is the platter size in a multi-wafer system or the wafersize in a single wafer system.

In some embodiments, the substrate can include a Group III metalnitride. The Group III metal nitride can include GaN, InN, AlN, orcombinations thereof. The substrate can include a Group III metalnitride growing at a rate of at least 3 micrometer/hour, at a rate of atleast 4 micrometers/hour, or at least about 8 micrometers/hour. Thesubstrate can also include an n-type dopant, such as germanium.

Another embodiment of the disclosure can be a method for growing GroupIII metal nitrides, wherein the method includes the steps of generatinga plasma in a remote plasma chamber; flowing the plasma through agas-conductance barrier and into a growth chamber; and growing a groupIII metal nitride product on a substrate in the growth chamber. Thegrowth rate of the group III metal nitride product can be greater than 1micrometer/hour, greater than 3 micrometer/hour, greater than 4micrometers/hour, or greater than 8 micrometers/hour.

In some embodiment, the plasma can contain a combination of nitrogen andan inert gas. The inert gas can be selected from helium, neon, argon,xenon, and krypton, preferably from neon, argon and xenon. The inert gascan be selected from the group consisting of argon and xenon. The plasmacan include nitrogen gas and argon. In some embodiment, the nitrogen toinert gas ratio can be 1:20 to 20:1, or 1:1 to 10:1.

In the method, the nitrogen gas flow of the plasma can be at least 3sccm based on a 2 inch diameter substrate. The nitrogen gas flow can bescaled to large systems based on the equation SCCM=(GR)*(AREA)/SCALE,where GR is the growth rate in μm/hour, AREA is the size of the targetin square inches, and SCALE is a factor with units (μm inches{circumflexover ( )}2)/(SCCM-hour). SCALE can be between 0.1 and 50, between 1 and50, or between about 1 and 20.

In some embodiments, P_(P) can be at least 0.1 mTorr, at least 1 mTorr,at least 10 mTorr, or at least 100 mTorr. In some embodiments, the P_(g)can be less than about 0.1 mTorr, or less than about 0.05 mTorr. In oneexample, P_(P) can be at least 5 mTorr, and P_(g) can be less than about0.1 mTorr.

In some embodiments, the gas-conductance barrier can have a conductancevalue of at least about 5 L/sec, 7 L/sec or 10 L/sec. The conductancevalue can be scaled by r², where r is the platter size in a multi-wafersystem or the wafer size in a single wafer system.

In some embodiments, the method can include growing a group III metalnitride including GaN, InN, AlN, or combinations thereof. The Group IIImetal nitride can be grown at a rate of at least 3 micrometer/hour, at arate of at least 4 micrometers/hour, or at least about 8micrometers/hour.

In some embodiments, the method can further include doping the Group IIImetal nitride with an n-type dopant, including with germanium.

An embodiment of the disclosure can include a method for increasing thegrowth rate of group III nitrides in plasma-assisted MBE, wherein anitrogen plasma can be seeded with an inert gas, and the nitrogen flowrate can be at least 5 sccm. The nitrogen gas flow can be scaled tolarge systems based on the equation SCCM=(GR)*(AREA)/SCALE, where GR isthe growth rate in μm/hour, AREA is the size of the target in squareinches, and SCALE is a factor with units (μm inches{circumflex over( )}2)/(SCCM-hour). SCALE can be between 0.1 and 50, between 1 and 50,or between about 1 and 20. The inert gas can be selected from the groupconsisting of helium neon, argon and xenon. In some embodiments, theratio of nitrogen to inert gas can be at least 5:1, or at least 10:1.The ratio of nitrogen to inert gas can be between about 5:1 to about20:1.

An embodiment of the disclosure can include, in method for growing groupIII metal nitrides by plasma-assisted MBE using nitrogen gas in theplasma, the improvement comprising reducing the ion content of theplasma by increasing the pressure of the plasma to at least about 1mTorr. The method can further include adding an inert gas to thenitrogen plasma, the inert gas selected from the group consisting ofhelium, neon, argon and xenon, or can be argon. The ratio of the inertgas can be at least about 5:1, or at least about 10:1. The ratio ofnitrogen to inert gas can be between about 5:1 to about 20:1. Thenitrogen flow rate can be at least 5 sccm, and can be scaled based onthe equation SCCM=(GR)*(AREA)/SCALE, as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate growth rates of gallium nitride under variousconditions, in accordance with an exemplary embodiment of thedisclosure.

FIGS. 2A and 2B illustrate gallium nitride surfaces, in accordance withan exemplary embodiment of the disclosure.

FIGS. 3A and 3B illustrate plasma emission spectra of the plasma gas, inaccordance with an exemplary embodiment of the disclosure.

FIG. 4 illustrates ion reductions based on probe current for varyingamounts of pressure and gas combinations, in accordance with anexemplary embodiment of the disclosure.

FIGS. 5A and 5B illustrate the molecular nitrogen and atomic nitrogenplasma emission intensities with changing pressure and gas combinations,in accordance with an exemplary embodiment of the disclosure.

FIG. 6 illustrates electron concentration and mobility for germaniumdoping of a gallium nitride under various conditions, in accordance withan exemplary embodiment of the disclosure.

FIG. 7 illustrates a table of operating conditions for the presentinvention in accordance with an exemplary embodiment of the disclosure.

FIG. 8 illustrates a table of operating conditions for the presentinvention in accordance with an exemplary embodiment of the disclosure.

DETAILED DESCRIPTION

Although preferred embodiments of the disclosure are explained indetail, it is to be understood that other embodiments are contemplated.Accordingly, it is not intended that the disclosure is limited in itsscope to the details of construction and arrangement of components setforth in the following description or illustrated in the drawings. Thedisclosure is capable of other embodiments and of being practiced orcarried out in various ways. Also, in describing the preferredembodiments, specific terminology will be resorted to for the sake ofclarity.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise.

Also, in describing the preferred embodiments, terminology will beresorted to for the sake of clarity. It is intended that each termcontemplates its broadest meaning as understood by those skilled in theart and includes all technical equivalents which operate in a similarmanner to accomplish a similar purpose.

Ranges can be expressed herein as from “about” or “approximately” oneparticular value and/or to “about” or “approximately” another particularvalue. When such a range is expressed, another embodiment includes fromthe one particular value and/or to the other particular value.

By “comprising” or “comprising” or “including” is meant that at leastthe named compound, element, particle, or method step is present in thecomposition or article or method, but does not exclude the presence ofother compounds, materials, particles, method steps, even if the othersuch compounds, material, particles, method steps have the same functionas what is named.

It is also to be understood that the mention of one or more method stepsdoes not preclude the presence of additional method steps or interveningmethod steps between those steps expressly identified. Similarly, it isalso to be understood that the mention of one or more components in adevice or system does not preclude the presence of additional componentsor intervening components between those components expressly identified.

The disclosure includes a technique for extremely high growth rates inPAMBE growth of Group III metal nitrides, including for example GaN,while maintaining a smooth surface microstructure. By adding pumpingcapacity and a minor modification to the plasma source, a broad range ofgrowth rates can be achieved, while a mixed chemistry plasma shows afurther enhancement of the growth rate. Suppression of ion content canbe achieved by higher plasma pressure and through argon dilution. Agrowth rate of 9.8 μm/hour in an exemplary system represents more thanan order of magnitude increase compared to traditional growth rates inMBE, and even exceeds those commonly used in MOCVD.

The disclosure includes a PAMBE system that has a growth chamber, aremote plasma chamber, and a gas conductance barrier separating thegrowth chamber and the plasma chamber. The growth chamber operates at apressure P_(g), and the plasma chamber operates at a pressure P_(p).However, unlike previous systems, the gas conductance barrier, oraperture, provides a much higher conductance of gas flow between the twochambers. The gas conductance barrier can be at least 3 times theconductance of a traditional system, at least 4 times the conductance ofa traditional system, or at least 5 times the conductance of atraditional system. By increasing the conductance of the barrier, alarger flow rate of gas into the plasma can be achieved, which leads tothe increased growth rates. A pressure differential of the two chambersshould still be maintained in the system.

One embodiment of the disclosure then includes the pressure of theplasma chamber, designated P_(p). The plasma chamber pressure can be atleast about 0.1 mTorr, at least about 1 mTorr, at least about 10 mTorr,at least about 50 mTorr, at least about 100 mTorr, or at least about 250mTorr. An upper limit of the plasma chamber pressure can depend on thenature of the plasma chamber and the RF power used. However, one ofordinary skill would understand that the upper limit could be at apressure sufficient to maintain the plasma in a “bright mode” (oftenreferred to as inductive mode), i.e. before it switched to a “dim mode”(often referred to as capacitive mode) or fully extinguished. The plasmachamber pressure could therefore be less than 3 Torr, less than 2 Torr,or less than about 1 Torr.

One embodiment of the disclosure then includes the pressure of thegrowth chamber, designated P_(g). The growth chamber pressure can beless than P_(P), less than about 0.1 mTorr, less than about 0.08 mTorr,less than about 0.07 mTorr, or less than about 0.05 mTorr. The growthchamber pressure can be greater than about 0.005 mTorr.

The gas flow in this disclosure can be maintained at a rate high enoughto maintain the plasma and drive preferred grow rates. One of ordinaryskill would appreciate that the flow (Q, in torr-liters/second),conductance (C, in liters/second) and pressures (P_(P) and P_(g), intorr) are governed by the relationship Q=C(P_(P)−P_(g)), oftenapproximated by Q=CP_(P), when P_(P)>>P_(g). In an exemplary system,based on a 2 inch target, the nitrogen gas flow rate can be greater than3 sccm, greater than 4 sccm, or greater than 5 sccm. The nitrogen gasflow rate can be greater than 8 sccm, greater than 9 sccm or greaterthan 10 sccm. The nitrogen gas flow rate can be less than 50 sccm, lessthan 45 sccm, or less than 40 sccm in the exemplary system for a 2 inchtarget. The nitrogen gas flow rate can be between about 3 sccm and about40 sccm, between about 3 sccm and about 35 sccm, between about 8 andabout 35 sccm, or between about 10 and about 35 sccm, in an exemplarysystem for a 2 inch target.

One of skill in the art will recognize that nitrogen gas flow ratesdescribed above, while still above the traditional range of less than 1to about 3 sccm in a similar system, will depend entirely on the size ofsystem in which the gas is being applied to a larger system would havean inherently larger flow rate. Moreover, several practical aspects ofPAMBE add some error to the value, including particularly the amount ofspray past the substrate, the net desorption (non-sticking Nitrogen) ofNitrogen off the substrate and the efficiency of the plasma ingenerating reactive species. Nonetheless, the flow rates above can bescaled according to the system in which they are applied. The masstransport, SCCM in units of standard cubic centimeters per minute(sccm), of the system can be scaled according to the equationSCCM=(GR)*(AREA)/SCALE, where GR is the growth rate in μm/hour, AREA isthe size of the target in square inches, and SCALE is a factor withunits (μm inches{circumflex over ( )}2)/(SCCM-hour) that accounts forthe fact that not all the gas is incorporated in the growing film, somehaving missed the substrate, some desorbed from the substrate and alsoaccounting for the percent activation of the plasma. The SCALE can bebetween about 0.1 to 50, from about 1 to 50, from about 1 to 20, fromabout 1 to 10, from about 2 to 10, or from about 1 to about 5.

Another aspect of the disclosure is the gas conductance barrierseparating the plasma chamber from the growth chamber. The gasconductance barrier has a conductance associated with it which can varywith the size of the system. In an embodiment of the disclosure, theconductance of the barrier can be between about 1 L/sec and 100 L/sec.The conductance can be greater than about 2 L/sec, greater than about 3L/s, or greater than about 5 L/sec. The conductance can be greater thanabout 7 L/sec, greater than about 10 L/s, or greater than about 15L/sec. The conductance can be less than about 90 L/sec, less than about75 L/sec, or less than about 50 L/sec. In an embodiment, the conductancecan be from about 2 L/sec to about 90 L/sec, from about 3 L/sec to about75 L/sec, from about 4 L/sec to about 60 L/sec, or from about 5 L/sec toabout 50 L/sec. In an embodiment, the conductance can be from about 10L/sec to about 90 L/sec, from about 10 L/sec to about 75 L/sec, fromabout 10 L/sec to about 60 L/sec, or from about 10 L/sec to about 50L/sec. One of skill in the art will recognize that larger systems can bescaled for larger conductance values. That conductance value, like thegas flow, can increase at r² where r is the platter size in amulti-wafer system or the wafer size in a single wafer system.

As demonstrated in further detail below, a high growth rate and highquality of the group III metal nitride structures can be achieved usinga pure nitrogen plasma. However, as the pressure from the decreased gasflow decreases further, detrimental effects to the substrates can beobserved due to high energy species created when operating at lowernitrogen pressures. This ion content should be controlled in order toachieve high quality III-Nitrides. By including an inert gas flow intothe nitrogen stream, the ion content can be reduced, by either changingthe electron temperature of the plasma or in other cases, operating at ahigher plasma pressure where the mean free path between collisions isminimized, and even higher grow rates can be achieved. The last featureis likely due to an effective higher electron temperature of the plasmawhen the inert gas is present. Thus, an embodiment of the disclosureincludes a plasma that includes nitrogen gas and an inert gas. The inertgas can be helium, neon, argon xenon or krypton. The inert gas can beneon, argon or xenon. The inert gas can be argon or xenon, or argon orhelium, or can be argon or can be helium. The ratio of gas flows fornitrogen to inert gas can be from 99:1 to 1:99. The ratio for nitrogento inert gas can be from 20:1 to 1:20, from 10:1 to 1:10, or from 5:1 to1:5. The ratio for nitrogen to inert gas can be from 20:1 to 1:1. Theratio for nitrogen to inert gas can be from 20:1 to 3:2, or from 20:1 to2:1. In some embodiments, the nitrogen to inert gas ratio is greaterthan 3:2, greater than 2:1, greater than 3:1, greater than 4:1, orgreater than 5:1. As used here, the ratio is for the gas flows ofnitrogen and inert gas, e.g. sccm N₂:sccm inert gas. Uses of highlydilute nitrogen in the inert gas have the opposite effect, reducinggrowth rates, providing a net wider growth rate range.

The RF power can be any RF power used to generate a plasma. In anembodiment, the RF power can be between about 300 W and about 650 W,between about 350 W and about 650 W, or between about 350 W and about600 W. It is well known in the art that what is important about thesepowers is the power density not the absolute power. Thus, for largersystems, these powers scale with the volume of the plasma chamberwherein the present embodiment's current plasma chamber is ˜1 inchdiameter×4 inches long cylindrical.

For some embodiments described herein, the high growth rates can beachieved for substrate temperatures from about 300° C. to about 1400° C.The substrate temperature can be between about 300 and about 800° C.,between about 300 and about 650° C., between about 300 and about 1000°C., between about 500 and about 1200° C., between about 500 and about800° C., between about 600 and about 1000° C.

The disclosure herein can be used to rapidly grow Group III metalnitride structures. The group III metal nitride can be AlN, GaN, or InN,and mixed metal combinations thereof, such as InGaN, AlGaN, or InGaAlN.In some embodiments, the group III metal nitride can include InN or GaN,or can be GaN.

The disclosure herein includes growth rates of group III metal nitridessubstantially above traditional MBE rates. The growth rate can be atleast about 1 μm/hour, at least about 2 μm/hour, or at least about 3μm/hour. The growth rate can be at least about 5 μm/hour, or at leastabout 8 μm/hour.

Several different aspects of the disclosure can thus be discussed. Oneembodiment of the disclosure can include a system for performing PAMBE,wherein the system includes a growth chamber, which has a substratemounted therein; a plasma chamber separated from the growth chamber, anda gas conductance barrier separating the plasma chamber and the growthchamber. The pressures of the two chambers, the conductance of thebarrier, and the gases used to create the plasma are as described above.

Another embodiment can be a Group III metal nitride rapid growth PAMBEsystem. The system can have growth chamber that grows the group IIImetal nitride on a substrate mounted therein. The system can have anitrogen gas plasma chamber separated from the growth chamber, and a gasconductance barrier separating the plasma chamber and the growthchamber. The pressures of the two chambers, the conductance of thebarrier, and the gases used to create the plasma are as described above.The rapid growth can be at a rate of at least about 1 μm/hour, at leastabout 2 μm/hour, or at least about 3 μm/hour. The growth rate can be atleast about 5 μm/hour, at least about 7 μm/hour, at least about 8μm/hour, or at least about 9 μm/hour. The group III metal nitride can beAlN, GaN, or InN, and mixed metal combinations thereof, such as InGaN,AlGaN, or InGaAlN; preferably InN or GaN, or can be GaN.

The disclosure also provides for a method of growing group III metalnitrides, wherein the method comprises generating a plasma in a PAMBEsystem, flowing the plasma through a gas conductance barrier into agrowth chamber, and growing a group III metal nitride on a substrate inthe growth chamber. The growth rate of the group III metal nitride canbe greater than 1 μm/hour, greater than about 2 μm/hour, or greater thanabout 3 μm/hour. The growth rate can be at least about 5 μm/hour, or atleast about 8 μm/hour.

The plasma in the method can comprise a nitrogen gas, or can comprise acombination of nitrogen and an inert gas mixture. The inert gas, theratio of nitrogen to inert gas, and the flow rate of the gas can be asdescribed above.

The method includes a pressure in the plasma chamber, which can bedefined at P_(p), and is as described above. Similarly, the methodincludes a pressure in the growth chamber, P_(g), as described above.The substrate in the growth chamber can be at a substrate temperature asdescribed above.

Another embodiment of the disclosure includes a method of reducing theion content of a nitrogen plasma in an PAMBE system. The method includesincreasing the pressure of the plasma to at least 1 mTorr, or at least10 mTorr, or at least 100 mTorr. The method includes feeding an inertgas into the nitrogen plasma at a ratio of nitrogen to inert gas of 99:1to 1:99. The ratio for nitrogen to inert gas can be from 20:1 to 1:20,from 10:1 to 1:10, or from 5:1 to 1:5. The ratio for nitrogen to inertgas can be from 20:1 to 1:1. The ratio for nitrogen to inert gas can befrom 20:1 to 3:2, or from 20:1 to 2:1. In some embodiments, the nitrogento inert gas ratio is greater than 3:2, greater than 2:1, greater than3:1, greater than 4:1, or greater than 5:1. The method of reducing ioncontent in the nitrogen plasma can include forming the plasma in aplasma chamber, flowing the plasma through a gas conductance barrierinto a growth chamber, and growing a group III metal nitride on asubstrate in the growth chamber. The growth rate of the group III metalnitride can be greater than 1 μm/hour, greater than about 2 μm/hour, orgreater than about 3 μm/hour. The growth rate can be at least about 5μm/hour, or at least about 8 μm/hour.

Some exemplary embodiments of the systems and the methods can then bedescribed. The system and/or method can include a nitrogen:inert gasmixture of between 10:1 and 1:1, a growth rate of a group III metalnitride of at least 3 μm/hour, and a metal nitride comprising GaN. Thesystem and/or method can include a nitrogen:inert gas mixture of between10:1 and 2:1, a growth rate of a group III metal nitride of at least 3μm/hour, and a metal nitride comprising GaN. The system or method caninclude a nitrogen gas flow rate for a 2 inch substrate of at leastabout 7 sccm, at least about 10 sccm, or at least about 15 sccm. Thegrowth rate of a group III metal nitride can be at least 5 μm/hour, atleast 6 μm/hour, at least 7 μm/hour, or at least 8 μm/hour. The ioncontent based on a probe current can be reduced by at least about 15%,at least about 25%, at least about 33%, at least about 50%, at leastabout 66%, or at least about 75%.

The system and methods described above can also be used to add a n-typedopant in the PAMBE system to the rapidly growing group III metalnitride. The methods can include further doping a growing metal nitridewith an n-type dopant. The systems can include the metal nitride grownon the substrate, which further includes an n-type dopant. The n-typedopant be any metal applicable to the PAMBE process. The n-type dopantcan preferably be germanium.

Utilizing a modified nitrogen plasma source, plasma assisted molecularbeam epitaxy (PAMBE) has been used to achieve higher growth rates inGaN. A modified conductance aperture plate, combined with highernitrogen flow and added pumping capacity, resulted in dramaticallyincreased growth rates up to 8.4 μm/hour using 34 sccm of N₂ while stillmaintaining acceptably low operating pressure. It was further discoveredthat argon could be added to the plasma gas to enhance growth rates upto 9.8 μm/hour which was achieved using 20 sccm of N₂ and 7.7 sccm Arflows at 600 W RF power. A remote Langmuir style probe employing theflux gauge was used to indirectly measure the relative ion content inthe plasma. The use of argon dilution at low plasma pressures resultedin a dramatic reduction of the plasma ion current by more than half,while high plasma pressures suppressed ion content regardless of plasmagas chemistry. Moreover, different trends are apparent for the molecularand atomic nitrogen species generated by varying pressure and nitrogencomposition in the plasma. (See FIG. 4.) Argon dilution resulted innearly an order of magnitude achievable growth rate range from 1 μm/hourto nearly 10 μm/hour. Even for films grown at more than 6 μm/hour, thesurface morphology remained smooth showing clear atomic steps with rootmean square roughness less than 1 nm. Due to the low vapor pressure ofSi, Ge was explored as an alternative n-type dopant for high growth rateapplications. Electron concentrations from 2.2×10¹⁶ to 3.8×10¹⁹ cm⁻³were achieved in GaN using Ge doping, and unintentionally doped GaNfilms exhibited low background electron concentrations of just 1-2×10¹⁵cm⁻³. The highest growth rates resulted in macroscopic surface featuresdue to Ga cell spitting which is an engineering challenge still to beaddressed. Nonetheless, the dramatically enhanced growth ratesdemonstrate great promise for the future of III-nitride devices grown byPAMBE.

Examples

Substrate, Apparatus, and Growth Technique

All films, unless otherwise specified, were grown in a Riber 32 MBEsystem with a base pressure of approximately 8×10⁻¹¹ Torr. Thesubstrates were single-side polished 1×1 cm sapphire, 1×1 cm GaNtemplates, or 2″ sapphire wafers. All substrates were sputter coatedwith 2 μm of tantalum on the back side for heat absorption andspreading. After cleaning in a 4:1 solution of H₂SO₄:H₂O₂, thesubstrates were loaded into an introductory chamber on an indium-freemolybdenum block, and then outgassed at 150° C. for 20 minutes. Thesamples were then transferred to the growth chamber and were outgassedfor an additional 10 minutes at 600° C. for the GaN template substratesor 850° C. for the sapphire substrates. Any substrate temperatureslisted herein were those measured by thermocouple. For all film growths,nitrogen was supplied by a Veeco UNI-Bulb RF plasma source, with severalmodifications which will be described later. The gas flow and RF powerwere varied throughout the study to achieve the desired surfaceconditions, and also to explore their effects on growth rate. Aluminumwas supplied by a cold-lip effusion cell, gallium was supplied by astandard effusion cell and/or a Veeco dual-filament SUMO cell, andgermanium was supplied by a standard effusion cell for n-type doping.The total Ga fluxes used in this study varied from ˜8×10⁻⁷ Torr beamequivalent pressure (BEP) up to ˜3.6×10⁻⁶ Torr BEP.

The growth techniques employed herein were a combination of traditionalMBE and a shuttered variation of MBE called Metal-Modulated Epitaxy(MME). For the sapphire substrates, a low-temperature nitridation wasperformed for 1 hour at 200° C., followed by a 10 nm AlN nucleationlayer under N-rich conditions (III/V≈0.7) at 700° C. Then, a thicker AlNbuffer layer was grown using MME at 850° C. under Al-rich conditions(III/V≈1.5), while the substrate temperatures during GaN growth weretypically 600 to 650° C. This MME technique has been shown to achieveextremely high p-type doping, control phase separation in InGaN, andresult in excellent uniformity and material utilization due to thetemperatures substantially lower than traditional PAMBE.

The film growth was monitored in situ by reflection high energy electrondiffraction (RHEED), and the plasma emission spectra were monitoredthrough the rear window of the plasma source looking through thepyrolytic boron nitride (PBN) bulb using an Ocean Optics HR-2000+ESspectrometer in the wavelength range of 200-1100 nm. Post-growthstructural analysis was performed by x-ray diffraction (XRD) using aPhilips X'Pert Pro MRD, and the surface morphology of the films wascharacterized by a Veeco Dimension 3100 atomic force microscope (AFM)operated in tapping mode. Film thicknesses were verified by eithercontact profilometry or spectroscopic ellipsometry. Hall Effect wasemployed to determine the electrical properties of select films using a0.3 T fixed magnet or a custom Hall effect system with a 1.5 Telectro-magnet.

Modifications to Plasma and Chamber

The overall pumping system for the growth chamber consisted of one CTI-8cryo pump, two CTI-10 cryo pumps all used during growth, and one ionpump and a titanium sublimation pump used when not growing. With theadded cryo pumps, the pumping capacity during growth was more thantripled compared to the previous single cryo pump configuration. Thisincreased capacity resulted in a substantial reduction in the chamberpressure during growth, never exceeding 10⁻⁴ Torr even with more than 30standard cubic centimeters per minute (sccm) of N₂ flow. The lowerchamber pressure reduces wear on effusion cell filaments, and couldserve to decrease scattering of the incident flux to improve growthrates.

In addition to the added pumping capacity, the plasma source itself wasalso slightly modified. The original plasma aperture (FIG. 7) wasreplaced with a custom 5.6× higher conductance aperture (FIG. 8) inorder to allow for higher gas flow while maintaining the correct plasmamode. With the original aperture, the plasma switched to a dim moderesulting from excessive back pressure behind the aperture with anitrogen flow of just 3.25 sccm, but remained lit in the bright modewith flow less than 1 sccm. Using the new high-conductance aperture, theplasma required substantially higher nitrogen flow of at least 3 sccm toremain ignited, but continued in the bright mode up to the maximumtested nitrogen flow of 34 sccm.

Growth Rate of GaN Using Mixed Nitrogen and Argon Gas

In order to initially characterize the plasma source, the GaN growthrate was determined using post-growth thickness measurements combinedwith in situ estimation of III/V flux ratios by transient RHEEDanalysis. The RHEED and post growth methods matched to less than 5%accuracy in all cases. To avoid desorption of the Ga flux, the samplesused for growth rate determination were grown at a low substratetemperature of 600° C. under metal-rich conditions. The nitrogen flowwas initially varied from 3 sccm to 17 sccm with a fixed power of 350 W,and then the plasma power was varied from 350 W to 500 W with a fixednitrogen flow of 15 sccm. This resulted in growth rates of 1.5 μm/hourto 6.1 μm/hour. Using pure nitrogen at 500 W RF power and increasing theflow to 34 sccm using a second mass flow controller yielded a growthrate of 8.4 μm/hour.

To assist in the initial striking of the plasma after baking the system,a light flow of argon gas was added which is easier to ignite than purenitrogen. After the plasma ignited, the argon flow was cut off and theplasma transitioned to pure nitrogen. However, it was observed that thisprocedure resulted in an increase in the active nitrogen flux for filmsgrown immediately after the use of argon. As such, another test wasperformed to determine the effect of argon on the growth rate forvarious conditions. From this test, it was discovered that the growthrate increased by approximately 20 to 50 percent at high and lownitrogen flows, respectively, with the addition of 7.7 sccm argon intothe plasma. FIGS. 1A and 1B show the overall growth rate as a functionof N₂ flow for various power conditions (A) or RF power for a N₂ flow of15 sccm (B). Growth rates range from approximately 1.5 μm/hour to 9.8μm/hour.

The maximum growth rate was achieved using 600 W RF power with 20 sccmnitrogen and 7.7 sccm argon flows, which is limited by our current massflow controller hardware configuration. This is not to be considered anultimate limitation in our embodiment as higher flow rates and thushigher growth rates could be achieved with higher mass flow controllers.This condition resulted in an extremely high growth rate of 9.8 μm/hourwhich is, to our knowledge, the highest ever reported for GaN grown byMBE. Under this same condition using 20 sccm nitrogen without argon, thegrowth rate dropped to 8.2 μm/hour.

While the demonstration of high growth rates is beneficial for futureapplications, the films should also remain sufficiently high quality inorder to be useful. FIGS. 2A and 2B display a representative RHEED imageduring growth (A), as well as a 2×2 μm AFM image for a film grown at 6.1μm/hour at a substrate temperature of 600° C. (B). During the growth ofthese films under metal-rich conditions at 600° C. the RHEED patternsremained streaky and a 2×2 reconstruction was observed, indicating asmooth growth surface. The AFM image shows excellentdislocation-mediated step-flow morphology with clear atomic steps andspiral hillocks and a root mean square (RMS) roughness of just 0.8 nm.Such a surface structure is typical for high quality GaN grown by MBE,and is also seen at much lower growth rates. This observation confirmsthat the 2D growth mode is preserved even at growth rates more than anorder of magnitude higher than are commonly used in GaN MBE.

Plasma Quality/Characterization

Plasma emission spectra were compared using a simple fiber-coupledspectrometer on the rear plasma window. The spectra in FIGS. 3A and 3Brepresent 20 sccm pure N₂ (black), a mixed plasma using 20 sccm N₂ with7.7 sccm Ar (red), and finally a pure 7.7 sccm Ar (blue) using 350 W ofRF power in all cases. The full wavelength range is displayed (A) alongwith a zoom (B) of the region 550 nm to 690 nm corresponding to excitedmolecular nitrogen (N₂*) which has been shown to directly affect growthrate of III-nitrides in plasma MBE.

From FIG. 3B it is clear that the addition of argon had no meaningfuleffect on the excited molecular nitrogen bands, in contrast to otherfindings using a mixed N₂/Ar plasma. However, the intensities of theargon-related emission lines were dramatically reduced by a factor of3-4× in the mixed plasma compared to the pure argon plasma. Thus, it ispossible that the reduction in argon-related emission intensity in themixed plasma could be due to the transfer of kinetic energy from theargon species to the nitrogen species, enhancing the formation of otheractive nitrogen species besides molecular species and increasing thegrowth rate.

To further explore the effects of argon in the plasma, the manipulatorwas put into the flux measurement position while measuring the collectorcurrent using a Keithley 6517B electrometer with the collector bias heldat 0V. During the measurement, the grid voltage was set to 0V and theflux gauge filament was turned off with the collector shielded by earthground except for a small opening facing the plasma source. This ensuresthat the only ionized/charged species reaching the collector were thosecoming from the plasma source itself and or species created near thecollector of the ion gauge. As such, the collector functioned similar toa Langmuir probe where the collector current is related, albeitindirectly, to the ion content of the plasma as discussed below. FIG. 4shows the resulting probe current from this experiment at 300 W RF poweras a function of the nitrogen/argon plasma compositions as well as theapproximate pressure inside the plasma bulb (i.e. behind the apertureplate) as calculated by the mass flow rate and the theoreticalconductance of the aperture plate.

It should be first emphasized that this experimental configuration withzero bias will result in the collection of both electrons and ions, andis thus related to but not equal to the ion content. With a positivebias applied to the grid, positively charged primary ions should berejected and negatively charged primary electrons attracted, or viceversa for a negative bias on the grid. However, in this experiment itwas found that the probe current was always negative with a magnitude ofat least 10 nA regardless of applied grid bias up to ±20V. This was nota measurement offset error as the probe current was less than 10 pAwhile the plasma was turned off. Thus, the only reasonable explanationfor this persistent large negative current is the presence of a highdensity of secondary electrons created by the primary high energyspecies (ions and/or electrons from the plasma) which overwhelm theprimary ion and electron currents from the plasma itself. Secondaryelectrons can be generated from nearby surfaces via impact ionization orphotoemission, the latter having been acknowledged as a complicatingfactor but discounted herein as has been done in prior more detailedcharacterization efforts. Because the trends in optical intensity(described later) do not follow the trends in probe current, the role ofphotoemission is assumed negligible in the present case. However, bothof these mechanisms (impact ionization and photoemission) suggest thatthe plasma produces sufficiently high energy primary species to causeionization and, thus, potential ionizing damage to the sample duringgrowth. Thus, we do not treat the measured collector currents as adirect measure of ion current but instead an impact ionization enhancedcurrent related to but not equal to ion current. Therefore regardless ofsign of the current, variations in the current are assumed to originatefrom changes in the high energy, potentially damaging, primary plasmaspecies.

From FIG. 4 it is clear that, in the lower pressure regime, the purenitrogen plasma results in a dramatic increase of more than 100% inprobe current compared to the pure argon plasma. Additionally, withproper dilution of the nitrogen with argon at lower pressure/flow, theprobe current, and thus inferred ion/electron primary flux, can bedramatically reduced in order to minimize any detrimental effect of highenergy species bombarding the sample surface and causing damage. Higherplasma pressure suppresses the ion content, making the ion contentrelatively unaffected by plasma composition. This observation of probecurrent shows two mechanisms to control plasma damage due to high energyspecies: in the lower flow regime by argon dilution and in the higherflow regime via pressure.

Shown in FIGS. 5A and 5B are the emission intensities at 300 W RF powerfor the 650 nm and 821 nm plasma emission lines relating to molecularnitrogen (A) and atomic nitrogen (B) transitions. The 650 nm emissionfrom excited molecular transitions in FIG. 5(a) shows a monotonicincrease in intensity with increasing nitrogen composition in theplasma, and also with increasing pressure. Interestingly, asubstantially different trend can be seen in the 821 nm atomic emissionin FIG. 5(b) where the intensity trends do not follow the 650 nmmolecular increases. At lower nitrogen compositions of 25% and 50%, theatomic nitrogen intensity increases with increasing pressure. However,above 50% nitrogen there is a peak in the atomic emission intensity at˜10 mTorr which then decreases at higher pressure. In the case of a purenitrogen plasma, the atomic emission intensity is nearly 55% higher at10 mTorr compared to 20 mTorr. The overall atomic nitrogen intensity ishighest for a diluted plasma with just 50% nitrogen at 20 mTorr.

These trends in the atomic nitrogen intensity can further explain theincreased growth rate with argon as described previously and as shown inFIG. 1(a). In the literature, there is no clear consensus which nitrogenspecies (atomic or molecular) is the primary contributor to growthduring PAMBE of III-nitrides. In this work, at the higher plasmapressures where the growth rate is highest, there is a substantial shiftin the trends of atomic nitrogen intensity compared to molecularnitrogen. In the high pressure/flow case, the dilution with ˜25-50%argon minimally reduces the molecular emission but substantiallyincreases the atomic emission intensity of the plasma. Thus, one wouldassume that the 20-25% increase in growth rate observed in FIG. 1(a)results from an enhancement in atomic nitrogen, not the traditionallyassumed excited molecular species. At lower plasma chamber pressures, 10mTorr or less, both the molecular and atomic nitrogen intensities arehighest using a pure nitrogen plasma. But, at higher pressures likethose used to achieve growth rates above 6 μm/hour, the atomic nitrogenis maximized for a mixed chemistry plasma with ˜50% argon dilution. Themaximum growth rate of 9.8 μm/hour reported here was achieved at apressure of ˜30 mTorr with a nitrogen composition of ˜72%. A morein-depth study is currently underway in order to explore the relativeeffects of atomic and molecular nitrogen and the various plasmaconditions on growth rate and structural/optical quality. But, from thispreliminary characterization of the plasma, it is clear that both theatomic and molecular nitrogen species are important to consider,especially at the extremely high growth rates reported herein.

In this study, the maximum growth rate achieved was not limited by theplasma source, but rather by the 20 sccm nitrogen mass flow controllerwhich indicates that even higher growth rates should be possible. Inaddition, the growth rate can be reduced by varying the nitrogen andargon at low flow rates. With pure nitrogen, the plasma extinguished ata flow of ˜3 sccm due to insufficient back-pressure. However, it couldbe sustained by using a combination of 1 sccm of nitrogen with 7.7 sccmof argon, resulting in more traditional growth rates closer to 1μm/hour. By lowering the argon content further, an additional reductionin growth rate might also be achievable to extend the range of possiblegrowth rates. The varied growth rate from ˜1 μm/hour up to 9.8 μm/hourrepresents nearly an order of magnitude achievable range of growth rateswhich can be tuned for rapid growth of thick buffer layers or preciselycontrolled quantum wells.

Doping with Ge vs Si

In order to achieve sufficient n-type doping at these elevated growthrates, some reconsideration of the Si doping method is needed.Typically, n-type doping in MOCVD growth using Si is limited to the lowor mid 10¹⁹ cm⁻³ range due to tensile stress and film cracking duringcool down, as well as morphological degradation at higher Siconcentrations. In MBE, due primarily to the lower substratetemperatures, this is less of an issue. Using MME, we have previouslyachieved extremely high Si doping with an electron concentration up to2.5×10²⁰ cm⁻³ and resistivity of 5×10⁻⁴ Ω-cm for a 900 nm thick filmwith RMS roughness of just 0.3 nm. This film was grown at ˜1 μm/hour,and the Si dopant cell required a temperature in excess of 1200° C.However, in order to achieve this high level of doping at elevatedgrowth rates, an even higher cell temperature would be required, pushingthe power limits of the cell and reducing effusion cell and/or cruciblelifetime.

To circumvent this doping challenge at high growth rates, the use of Gewas explored as an alternative. Compared to Si, Ge has approximately twoorders of magnitude higher vapor pressure for the same temperature. Or,to achieve the same vapor pressure as Si, a Ge cell could be run atapproximately 200° C. lower temperature. Ge has been shown to havesimilar activation energy to Si in GaN, and induces substantially lessstress on the crystal due to its closer atomic size match to the Ga atomit substitutes in the lattice. Given the lower temperature requirementswith similar activation, Ge would seem an excellent alternative for highgrowth rate GaN. The use of Ge was investigated by varying the Ge celltemperature from 760° C. to 1000° C. with a fixed growth rate and Gaflux. FIG. 6 shows electron concentration (black—lower line) andmobility (blue—upper line) as a function of Ge cell temperature.Sapphire substrates were used for this study to provide completeelectrical isolation and minimize the effect of surface contamination. A500 nm thick AlN buffer layer was grown by MME after low-temperaturenitridation. The resulting AlN, grown at nearly 2 μm/hour, showed (002)rocking curve (omega scan) full width at half maximum (FWHM) of around80 arcsec with RMS roughness of ˜0.5 nm. For the Ge-doped GaN grown at650° C., MME was employed using a 10 s open/10 s closed shuttermodulation with a III/V ratio of approximately 1.8, and both Ge and Gawere shuttered simultaneously. The plasma condition for these films wasfixed at 350 W RF power with 4 sccm N₂ flow, yielding a growth rate ofapproximately 2 μm/hour.

The electron concentration in FIG. 6 exhibits a highly predictableexponential dependence on the Ge cell temperature. The electronconcentrations achieved varied from 2.2×10¹⁶ cm⁻³ at a Ge celltemperature of 760° C. up to 3.8×10¹⁹ cm⁻³ at a Ge cell temperature of999° C. These low dopant cell temperatures leave substantial headroomfor high n-type doping at high growth rates while simultaneouslyreducing wear on the cell and crucible. As a baseline, severalunintentionally doped (UID) GaN films were grown by MME in order tocharacterize the background electron concentration. Since MBE grownfilms have background impurity concentrations inversely proportional togrowth rate, low background impurity concentrations are expected atthese high growth rates. Indeed, with the same conditions as discussedabove, the UID GaN films exhibited repeatable background electronconcentrations of 1-2×10¹⁵ cm⁻³ with resistivity on the order of 50-100Ω-cm. The mobility, shown in blue in FIG. 6, initially rises withincreasing Ge cell temperature then begins to fall once the electronconcentration exceeds ˜4×10¹⁸ cm⁻³. This trend, along with therelatively low overall mobility, can be explained in the context ofrelatively high dislocation density resulting from the immature natureof these initial high growth rate studies.

In these non-optimized 1.8 μm thick GaN films grown on AlN buffer layerson sapphire, the XRD GaN (002) and (102) omega rocking curve FWHM aretypically on the order of 250 arcsec and 1500 arcsec, respectively.Based on simple estimations from these two rocking curves, the threadingdislocation density in these Ge-doped films is expected to be on theorder of 5×10⁹ to 1×10¹⁰ cm⁻². With these relatively high dislocationdensities, the mobility is strongly affected by dislocation scatteringat low carrier concentrations. At higher carrier concentrations, thedislocation scattering is minimized due to charge screening, while atvery high doping concentrations the mobility again decreases due toionized impurity scattering. Thus, while the mobility in these Ge-dopedsamples is relatively low, it is well-described by physical models andlimited by dislocation density.⁴¹ It is expected that the mobilityshould be substantially higher when grown on lower dislocation densityGaN templates or freestanding GaN.

It is to be understood that the embodiments and claims disclosed hereinare not limited in their application to the details of construction andarrangement of the components set forth in the description andillustrated in the drawings. Rather, the description and the drawingsprovide examples of the embodiments envisioned. The embodiments andclaims disclosed herein are further capable of other embodiments and ofbeing practiced and carried out in various ways. Also, it is to beunderstood that the phraseology and terminology employed herein are forthe purposes of description and should not be regarded as limiting theclaims.

Accordingly, those skilled in the art will appreciate that theconception upon which the application and claims are based can bereadily utilized as a basis for the design of other structures, methods,and systems for carrying out the several purposes of the embodiments andclaims presented in this application. It is important, therefore, thatthe claims be regarded as including such equivalent constructions.

We claim:
 1. A plasma assisted MBE system comprising: a growth chamberhaving a substrate; a remote plasma chamber; and a gas-conductancebarrier separating the plasma chamber from the growth chamber; whereinthe system is configured such that a nitrogen gas flow of a plasma isscaled based on the equationSCCM=(GR)*(AREA)/SCALE wherein: GR is the growth rate in μm/hour; AREAis the size of the substrate in inches{circumflex over ( )}2; and SCALEis a factor with units (μm inches{circumflex over ( )}2)/(SCCM-hour)between 0.1 and
 50. 2. The system of claim 1, wherein the growth chamberhas a pressure P_(g), and the plasma chamber has a pressure P_(p), andthe gas conductance barrier allows the pressure P_(g) to be lower thatP_(P); and wherein P_(P) is at least 0.1 mTorr.
 3. The system of claim1, wherein SCALE is between 1 and
 20. 4. The system of claim 1, whereinthe growth chamber has a pressure P_(g), and the plasma chamber has apressure P_(p), and the gas conductance barrier allows the pressureP_(g) to be lower that P_(P); and wherein P_(P) is at least 100 mTorr.5. The system of claim 2, wherein P_(g) is less than about 0.1 mTorr. 6.The system of claim 1, wherein the plasma chamber contains plasmacomprising nitrogen and an inert gas mixture; and wherein the system isconfigured such that the nitrogen gas flow of the plasma is at least 3SCCM based on a 2 inch diameter substrate.
 7. The system of claim 1,wherein the plasma chamber contains plasma comprising nitrogen and aninert gas mixture; and wherein the nitrogen to inert gas ratio is 1:20to 20:1.
 8. The system of claim 1, wherein the plasma chamber containsplasma comprising nitrogen and an inert gas mixture; and wherein thenitrogen to inert gas ratio is 1:1 to 10:1.
 9. The system of claim 1,wherein the system is further configured such that the gas-conductancebarrier has a conductance value of at least about 5 L/sec.
 10. A methodfor growing a group III metal nitride product comprising: flowing aplasma comprising nitrogen and an inert gas from a remote plasma chamberthrough a gas-conductance barrier and into a growth chamber; and growinga group III metal nitride product on a substrate in the growth chamberat a growth rate of at least 3 μm/hour; wherein the nitrogen gas flow isscaled based on the equationSCCM=(GR)*(AREA)/SCALE wherein: GR is the growth rate in μm/hour; AREAis the size of the target in inches{circumflex over ( )}2; and SCALE isa factor with units (μm inches{circumflex over ( )}2)/(SCCM-hour)between 0.1 and
 50. 11. The method of claim 10, wherein the nitrogen gasflow of the plasma is at least 3 SCCM based on a 2 inch diametersubstrate.
 12. The method of claim 10, wherein the nitrogen to inert gasratio is 1:20 to 20:1.
 13. The method of claim 10, wherein the nitrogento inert gas ratio is 1:1 to 10:1.
 14. The method of claim 10, whereinthe growth chamber has a pressure P_(g), the plasma chamber has apressure P_(p), and the gas conductance barrier allows the pressureP_(g) to be lower that P_(P); and wherein P_(p) is at least 0.1 mTorr.15. The method claim 10, wherein the growth chamber has a pressureP_(g), the plasma chamber has a pressure P_(p), and the gas conductancebarrier allows the pressure P_(g) to be lower that P_(P); and whereinP_(p) is at least 100 mTorr.
 16. The method of claim 10, wherein thegrowth chamber has a pressure P_(g), the plasma chamber has a pressureP_(p), and the gas conductance barrier allows the pressure P_(g) to belower that P_(P); and wherein P_(g) is less than about 0.1 mTorr. 17.The method of claim 10, wherein the gas-conductance barrier has aconductance value of at least about 5 L/sec.
 18. The method of claim 10,wherein the growth rate of the group III metal nitride product is atleast 8 micrometers/hour.
 19. The method of claim 10 further comprisingdoping the group III metal nitride product with an n-type dopant.
 20. Amethod for increasing the growth rate of group III nitrides inplasma-assisted MBE, wherein a nitrogen plasma is seeded with an inertgas; wherein the nitrogen flow rate is at least 5 SCCM; and wherein thenitrogen gas flow is scaled based on the equationSCCM=(GR)*(AREA)/SCALE wherein: GR is the growth rate in μm/hour; AREAis the size of the substrate in inches{circumflex over ( )}2; and SCALEis a factor with units (μm inches{circumflex over ( )}2)/(SCCM-hour)between 0.1 and
 50. 21. The method of claim 20, wherein SCALE is between1 and
 20. 22. The method of claim 20, wherein the inert gas is selectedfrom the group consisting of helium, neon, argon and xenon.
 23. Themethod of claim 20, wherein the ratio of nitrogen to inert gas is atleast 5:1.
 24. The method of claim 20, wherein the ratio of nitrogen toinert gas is at least 10:1.
 25. The method of claim 20, wherein theratio of nitrogen to inert gas is between about 5:1 to about 20:1. 26.In a method for growing group III metal nitrides by plasma-assisted MBEusing nitrogen gas in the plasma, the improvement comprising reducingthe ion content of the plasma by increasing the pressure of the plasmato at least about 1 mTorr; wherein the nitrogen gas flow is scaled basedon the equationSCCM=(GR)*(AREA)/SCALE wherein: GR is the growth rate in μm/hour; AREAis the size of the substrate in inches{circumflex over ( )}2; and SCALEis a factor with units (μm inches{circumflex over ( )}2)/(SCCM-hour)between 0.1 and
 50. 27. The method of claim 26 further including addingan inert gas to the nitrogen plasma, the inert gas selected from thegroup consisting of helium, neon, argon and xenon.
 28. The method ofclaim 26 further including adding an inert gas comprising argon to thenitrogen plasma.
 29. The method of claim 27, wherein the ratio ofnitrogen to inert gas is at least 5:1.
 30. The method of claim 26,wherein the nitrogen flow rate is at least 5 SCCM.
 31. The method ofclaim 26, wherein SCALE is between 1 and
 20. 32. The method of claim 10,wherein SCALE is between 1 and 20.